Physical post office locations do not accept PayPal at their registers. They only accept cash, checks, and standard credit/debit cards.
Add your desired stamps to the cart and select PayPal as your payment method at checkout. buy postage stamps with paypal
If you need stamps immediately and want to go to a physical retail store (like a grocery store, pharmacy, or office supply chain), you can still use your PayPal funds by utilizing these workarounds: Physical post office locations do not accept PayPal